Techniques for marine surveying include marine seismic surveying, in which geophysical data may be collected from below the Earth's surface. Seismic surveying has applications in mineral and energy exploration and production to help identify locations of hydrocarbon-bearing formations. Seismic surveying typically may include towing a seismic source in a body of water. One more “streamers” may also be towed through the water by the same or a different vessel. The streamers are typically cables that include a plurality of sensors disposed thereon at spaced apart locations along the length of each cable. Some seismic surveys locate sensors on ocean bottom cables or nodes in addition to, or instead of, streamers. The sensors may be configured to generate a signal that is related to a parameter being measured by the sensor. At selected times, the seismic source may be actuated to generate, for example, seismic energy that travels downwardly through the water and into the subsurface foimations. Seismic energy that interacts with interfaces, generally at the boundaries between layers of the subsurface formations, may be returned toward the surface and detected by the sensors on the streamers. The detected energy may be used to infer certain properties of the subsurface formations, such as structure, mineral composition and fluid content, thereby providing information useful in the recovery of hydrocarbons.
Most of the seismic sources employed today in marine seismic surveying are of the impulsive type, in which efforts are made to generate as much energy as possible during as short a time span as possible. The most commonly used of these impulsive-type sources are air guns that typically utilize compressed air to generate a sound wave. Other examples of impulsive-type sources include explosives and weight-drop impulse sources. Another type of seismic source that can be used in seismic surveying includes marine vibrators, including hydraulically powered sources, electro-mechanical vibrators, electrical marine vibrators, and sources employing piezoelectric or magnetostrictive material.
Marine vibrators typically generate vibrations through a range of frequencies in a pattern known as a “sweep” or “chirp.” For example, a sweep can be generated from 10-100 Hertz (“Hz”). The signal may then be correlated at the sensor to generate a pulse which should give the same result as using an impulsive-type source. Advantageously, the energy may be spread out with the sweep to provide a lessened environmental impact. If the marine vibrator is controllable and can handle arbitrary signals, signals may be created that use a direct spread spectrum that may have several advantages, such as low peak energy at any frequency at any time, decoding of the signal only by the user, and noise characteristics that may be difficult to distinguish from natural background noise, which may be advantageous in environmentally sensitive areas. In addition, a direct spread spectrum may allow the same bandwidth to be used with several marine vibrators at the same time. To increase signal level, the marine vibrators may be placed in an array. To be able to use arbitrary signals in a marine vibrator array, a feedback system may be used to produce repeatable signals and to reduce harmonics in the sweep. Feedback systems have been used, but can have drawbacks. For example, certain feedback systems may not be able to effectively reduce harmonics. If the sweep contains harmonics, it can add correlation noise when interfering with the fundamental tone during correlation for reservoir monitoring. Additionally, when sweeping at the same time with two or more marine vibrators at two or more different frequency ranges, the harmonics from the low frequency of one vibrator may interfere with the fundamentals of the other vibrator.